I. Introduction
This invention relates to new photoresist compositions. More particularly, this invention relates to positive working photoresist compositions containing a novel photoactive compound characterized by long term solubility in solution.
II. Description of the Prior Art
Photoresist compositions are well known in the art and described in numerous publications including DeForest, Photoresist Materials and Processes, McGraw-Hill Book Company, New York, 1975. Photoresists comprise coatings produced from solution or applied as a dry film which, when exposed to light of the proper wavelength, are chemically altered in their solubility to certain solvents (developers). Two types are known. The negative-acting resist is initially a mixture which is soluble in its developer, but following exposure to activating radiation, becomes insoluble in developer thereby defining a latent image. Positive-acting photoresists work in the opposite fashion, light exposure making the resist soluble in developer.
The positive-working photoresists typically comprise a light-sensitive compound in a film-forming polymer binder. The light-sensitive compounds, or sensitizers as they are often called, most frequently used are esters and amides formed from o-quinone diazide sulfonic and carboxylic acids. These esters and amides are well known in the art and are described by DeForest, supra, pp. 47-55, incorporated herein by reference. These light-sensitive compounds, and the methods used to make the same are all well documented and described in prior patents including German Patent No. 865,140 granted Feb. 2, 1953 and U.S. Pat. Nos. 2,767,092; 3,046,110; 3,046,112; 3,046,119; 3,046,121; 3,046,122; and 3,106,465, all incorporated herein by reference. Sultonic amide sensitizers that have been used in the formulation of positive-acting photoresists are shown in U.S. Pat. No. 3,637,384, also incorporated herein by reference. These materials are formed by the reaction of a suitable diazide of an aromatic sulfonyl chloride with an appropriate resin amine. Methods for the manufacture of these sensitizers and examples of the same are shown in U.S. Pat. No. 2,797,213, incorporated herein by reference. Other positive-working diazo compounds have been used for specific purposes. For example, a diazo compound used as a positive-working photoresist for deep U.V. lithography is Meldrum's diazo and its analogs is described by Clecak et al., "Technical Disclosure Bulletin," Volume 24, No. 4, September 1981, IBM Corporation, pp. 1907 and 1908, and o-quinone diazide compounds suitable for laser imaging as shown in U.S. Pat. No. 4,207,107. The aforesaid references are also incorporated herein by reference.
The resin binders most frequently used with the o-quinone diazides in commercial practice are the alkali-soluble phenol formaldehyde resins known as the novolak resins. Photoresists using such polymers are illustrated in U.K. Patent No. 1,110,017, incorporated herein by reference. These materials are the product of reaction of a phenol with formaldehyde, or a formaldehyde precursor, under conditions whereby a thermoplastic polymer is formed.
In the prior art, the above-described positive photoresists using novolak resins as binders are most often used as a mask to protect substrates from chemical etching and photo-engraving processes. For example, in a conventional process for the manufacture of printed circuit boards, a copper clad substrate is coated with a layer of a positive-working photoresist, exposed to actinic radiation to form a latent circuit image in the photoresist coating, developed with a liquid developer to form a relief image, and etched with a chemical etchant whereby unwanted copper is removed and copper protected by the photoresist mask is left behind in a circuit pattern. For the manufacture of printed circuit boards, the photoresist must possess chemical resistance, must adhere to the circuit board substrate, and for high density circuits, must be capable of fine-line image resolution.
Similar photoresists are also used in the fabrication of semiconductors. As in the manufacture of printed circuits, the photoresist is coated onto the surface of a semiconductor wafer and then imaged and developed. Following development, the wafer is typically etched with an etchant whereby the portions of the water bared by the development of the photoresist are dissolved while the portions of the wafer coated with photoresist are protected, thereby defining a circuit pattern. For use in the manufacture of a semiconductor, the photoresist must possess resistance to chemical etchants, must adhere to the surface of the semiconductor wafer, must possess good thermal properties and must be capable of very fine-line image resolution.
Photoresist properties, such as those identified above, have been improved by use of photoactive compounds of increased molecular weight and having a greater number of napthoquinone diazide moieties substituted onto the compound. This has been accomplished by use of polynuclear phenols having multiple hydroxyl groups. One method has used novolak oligomers as the backbone for formation of the photoactive compound. For example, reactions between 2,6-bis(hydroxymethyl)-p-cresol and alkyl monohydric phenols to form a novolak oligomer are known. These novolak oligomers may then be reacted with a naphthoquinone diazide sulfonyl halide to form a photoactive compound used to make a positive-working photoresist. See, for example, Japanese patent publication number 62-10646 (A) assigned to Kanto Chemical Company and published Jan. 19, 1987. An exemplary novolak oligomer disclosed in this reference has the following formula: ##STR2## Separately, reaction between 2,6-bis(hydroxymethyl)-p-cresol and resorcinol to form oligomeric polyhydroxyphenol novolaks were reacted with epichlorohydrin or the like to form a polyglycidic ether which is capable of combination with a curing agent to form a curable epoxy resin composition. This is disclosed in U.S. Pat. No. 4,614,826 incorporated herein by reference. The reaction product of 2,6-bis(hydroxymethyl)-p-cresol and resorcinol may be represented by the following formula: ##STR3##
The use of photoactive compounds that are condensation products of certain novolak oligomers condensed with a naphthoquinone diazide sulfonyl halide is disclosed in U.S. Pat. No. 4,992,596. These materials are formed by reacting the corresponding para-lower alkyl or halo-2,6-bis(hydroxymethyl)phenol with a polyhydroxy phenyl compound such as resorcinol or pyrogallol. This reaction is illustrated in the following reaction equation. ##STR4## where each X is selected from the group consisting of a hydroxyl group and a halogen group, Y is selected from the group consisting of lower alkyl having from 1 to 4 carbon atoms and halogen. The photoactive compound is formed by condensing the naphthoquinone diazide sulfonyl moiety with the oligomer to form a compound having the following structure ##STR5## where X and Y are as defined above and D is the naphthoquinone diazide sulfonic acid.
EPO published application No. 0 554 101 A1 discloses many polyhydric phenols used as ballast groups for photoactive compound formation. One such compound may be represented by the formula: ##STR6## where, in accordance with the published application, each of a, b, c, x, y and z is an integer of from 1 to 3.
It is known in the art that as the size of the photoactive compound increases and the number of naphthoquinone diazide sulfonic acid substituents increases, the long term solubility of the photoactive compound in the photoresist composition decreases. The problem is exacerbated by the fact that often the photoactive compound dissolves in the photoresist composition when freshly prepared, but during storage in its container, precipitates making the photoresist unsuitable for commercial use.